On Thu, Feb 13, 2020 at 11:05:13AM -0800, Ira Weiny wrote: > On Thu, Feb 13, 2020 at 11:01:57AM -0800, 'Ira Weiny' wrote: > > On Wed, Feb 12, 2020 at 02:49:48PM -0500, Jeff Moyer wrote: > > > Ira Weiny <ira.weiny@xxxxxxxxx> writes: > > > > > [snip] > > > > Given that we document the dax mount > > > option as "the way to get dax," it may be a good idea to allow for a > > > user to selectively disable dax, even when -o dax is specified. Is that > > > possible? > > > > Not with this patch set. And I'm not sure how that would work. The idea was > > that -o dax was simply an override for users who were used to having their > > entire FS be dax. We wanted to depreciate the use of "-o dax" in general. The > > individual settings are saved so I don't think it makes sense to ignore the -o > > dax in favor of those settings. Basically that would IMO make the -o dax > > useless. > > Oh and I forgot to mention that setting 'dax' on the root of the FS basically > provides '-o dax' functionality by default with the ability to "turn it off" > for files. Please don't further confuse FS_XFLAG_DAX and S_DAX. They are two separate flags with two separate behaviors: FS_XFLAG_DAX is a filesystem inode metadata flag. Setting FS_XFLAG_DAX on a directory causes all files and directories created within that directory to inherit FS_XFLAG_DAX. Mounting with -o dax causes all files and directories created to have FS_XFLAG_DAX set regardless of the parent's status. The FS_XFLAG_DAX can be get and set via the fs[g]etxattr ioctl. ------- S_DAX is the flag that controls the IO path in the kernel for a given inode. Loading a file inode into the kernel (via _iget) with FS_XFLAG_DAX set or creating a file inode that inherits FS_XFLAG_DAX causes the incore inode to have the S_DAX flag set if the storage device supports it. Files with S_DAX set use the dax IO paths through the kernel. The S_DAX flag can be queried via statx. --D > Ira > > > > > Ira > >
